The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present disclosure relates generally to oilfield cables and, in particular, to wireline cables, and methods of making and using such cables.
Several common problems encountered with wireline cables used in oilfield operations are related to armor wire strength members. Armor wire is typically constructed of cold-drawn plow ferritic steel coated with a zinc coating for corrosion protection. These armor wires provide the strength needed to raise and lower the weight of the cable and tool string and protect the cable core from impact and abrasion damage. Typical wireline cable designs consist of a cable core of one or more insulated conductors (packed in an interstitial filler in the case of multiple conductors) wrapped in cabling tape followed by the application of two armor wire layers. The armor wire layers are applied counterhelically to one another in an effort to minimize torque imbalance between the layers. In an effort to provide additional protection against impact, cut through, and abrasion damage, larger-diameter armor wires are typically placed in the outer layer. Due to shortcomings in these designs, torque imbalance between the armor wire layers continues to be an issue, resulting in cable stretch, cable core deformation and significant reductions in cable strength.
In pressurized wells, gas can infiltrate through gaps between the armor wires and travel along spaces existing between the inner armor wire layer and the cable core. Grease-filled pipes at the well surface provide a seal at the well surface. As the wireline cable passes through these pipes, pressurized gas can travel through the spaces among armor wires and the cable core. When the cable then passes over and bends over a sheave, the gas is released, resulting in an explosion and fire hazard.
In typical wireline cable designs, such as a wireline cable 10 shown in FIG. 1, outer armor wires 11 were sized larger than inner armor wires 12 in an effort to provide greater protection against impact, cut-through, and abrasion damage. One unintended effect of this design strategy is to increase torque imbalance. In those designs, the outer armor wires 11 carry roughly 60% of the load placed on the cable. This causes the outer armor wires 11 to straighten slightly when the cable is under tension, which in turn causes the cable core 13 to stretch and the inner armor wires 12 to be wound more tightly around the cable core. The outer armor wires 11 and inner armor wires 12 may come into point-to-point contact which wears away the protective zinc layer leading to premature corrosion. The cable core 13 can also be damaged as it deforms into the interstitial spaces between the inner armor wires 12. Additionally, because the outer armor wires 11 are carrying the bulk of the load, they are more susceptible to breaking if damaged, thereby largely negating any benefits of placing the larger armor wires in the outer layer.
Under tension, the inner and outer armor wires (which are applied at opposite lay angles) tend to rotate in opposite directions as shown by arrows 14 and 15 respectively as shown in FIG. 1. Because the larger outer armor wires 11 are dominant, the outer armor wires tend to open, while the inner armor wires 12 tighten, causing torque imbalance problems. To create a torque-balanced cable, the inner armor wires would have to be somewhat larger than the outer armor wires. This configuration has been avoided in standard wireline cables in the belief that the smaller outer wires would quickly fail due to abrasion and exposure to corrosive fluids. Therefore, larger armor wires have been placed at the outside of the wireline cable, which increases the likelihood and severity of torque imbalance.
Torque for a layer of armor wire can be described in the following equation.Torque=¼T×PD×sin 2α
Where: T=Tension along the direction of the cable; PD=Pitch diameter of the armor wires; and α=Lay angle of the wires.
Pitch diameter (the diameter at which the armor wires are applied around the cable core or the previous armor wire layer) has a direct effect on the amount of torque carried by that armor wire layer. When layers of armor wire constrict due to cable stretch, the diameter of each layer is reduced numerically the same. Because this reduction in diameter is a greater percentage for the inner layer of armor wires 12, this has a net effect of shifting a greater amount of the torque to the outer layer of armor wires 11.
In high-pressure wells, the wireline 10 is run through one or several lengths of piping 16 packed with grease to seal the gas pressure in the well while allowing the wireline to travel in and out of the well (see FIG. 2). Armor wire layers have unfilled annular gaps between the armor wire layers and the cable core. Under well conditions, well debris and the grease used in the risers can form a seal over the armor wires, allowing pressurized gas to travel along the cable core beneath the armor wires. Pressurized gas from the well can infiltrate through spaces between the armor wires and travel upward along the gaps between the armor wires and the cable core upward toward lower pressure. Given cable tension and the sealing effects of grease from the risers and downhole debris coating the armor wire layers, this gas tends to be held in place as the wireline travels through the grease-packed risers. As the wireline 10 bends when passing over the upper sheave 17 (located above the risers), the armor wires tend to spread apart slightly and the pressurized gas 18 is released. This released gas 18 becomes an explosion hazard (see FIG. 3).
It is desirable, therefore, to provide a cable that overcomes the problems encountered with wireline cable designs.
The disclosed designs minimize the problems described above by:
Placing layers of soft polymer between the inner armor wires and the cable core and between the inner and outer armor wire layers; and
Using larger-diameter armor wires for the inner layer than for the outer layer.
The polymeric layers provide several benefits, including:
Eliminating the space along the cable core and the first layer of armor along which pressurized gas might travel to escape the well;
Eliminating the space into which the cable core might creep and deform against the inner armor wires;
Cushioning contact points between the inner and outer armor wires to minimize damage from armor wires rubbing against each other;
Filling space into which the inner armor wire might otherwise be compressed, thereby minimizing cable stretch; and
Filling space into which the inner armor wire might otherwise be compressed, thereby minimizing the above-described effect of shifting torque to the outer armor wire layer when the diameters of both the inner and outer armor wire layers are decreased by the same amount.
Torque balance is achieved between the inner and outer armor wire layers by placing larger wires in the inner layer. As explained below, this allows the majority of the load to be carried by the inner armor wires. While in traditional armor wire configurations, the outer wires ended up carrying approximately 60 percent of the load and the inner wires approximately 40 percent. By placing the larger armor wires in the inner layer, the proportions of load can be more or less reversed, depending on individual cable design specifications.
The designs place soft thermoplastic polymer layers over the cable core and between the inner and outer armor wire layers and reconfigure the sizes of armor wires used such that larger armor wires are placed in the inner layer. As an option, these designs may utilize solid armor wires in the inner layer and stranded armor wires in the outer layer. These design changes result in a more truly torque-balanced cable that is sealed against intrusion and travel of pressurized gas. These designs may also have an outer layer of polymer to create a better seal at the well surface.